Method for heat-treating a manganese steel product and manganese steel product

Abstract

A method for heat treating a manganese steel product whose alloy comprises: a carbon fraction (C) between 0.09 and 0.15 wt. %, and a manganese fraction (Mn) in the range of 3.5 wt. %≤Mn≤4.9 wt. %, the method comprising: performing a first annealing process (S 4.1 ) with the substeps heating (E 1 ) the steel product to a first holding temperature (T 1 ), which lies above 780° C., holding (H 1 ) the steel product during a first time period (Δ 1 ) at the first holding temperature (T 1 ), cooling (A 1 ) the steel product, performing a second annealing process (S 4.2 ) with the substeps heating (E 2 ) the steel product to a holding temperature (T 2 ), which lies above 630° C. and below 660° C., holding (H 2 ) the steel product during a second time period (Δ 2 ) at the holding temperature (T 2 ), cooling (A 2 ) the steel product.

The invention claimed is: 
     
       1. Method for heat treating a manganese steel product: the method comprising the following steps:
 providing a steel product, selected from the group consisting of a hot-rolled manganese steel product and a cold rolled manganese steel product, whose alloy comprises: 
 a carbon fraction (C) between 0.09 and 0.15 wt. %, and 
 a manganese fraction (Mn) in the range of 4.0 wt. %≤Mn≤4.9 wt. %, and 
 fractions of bainite microstructure, 
 performing a first annealing process (S 4 . 1 ) with the following substeps
 heating (E 1 ) the steel product to a first holding temperature (T 1 ), which lies above 780° C., 
 holding (H 1 ) the steel product during a first time period (Δ 1 ) at the first holding temperature (T 1 ) thereby allowing austenite (v) formation, 
 cooling (A 1 ) the steel product, 
 
 performing a second annealing process (S 4 . 2 ) with the following substeps
 heating (E 2 ) the steel product to a holding temperature (T 2 ), which lies above 630° C. and below 660° C., 
 holding (H 2 ) the steel product during a second time period (Δ 2 ) at the holding temperature (T 2 ) thereby allowing the formation of the two phases ferrite and austenite, 
 cooling (A 2 ) the steel product, 
 wherein the cooling (A 1 ; A 2 ) of the steel product during the first annealing process (S 4 . 1 ) and during the second annealing process (S 4 . 2 ) is carried out at a cooling rate which lies between 25 Kelvin/second and 200 Kelvin/second, 
 and wherein the second annealing process (S 4 . 2 ) is carried out subsequently to the first annealing process (S 4 . 1 ). 
 
 
     
     
       2. The method according to  claim 1  wherein the first cooling of the steel product (A 1 ) and the second cooling of the steel product (A 2 ) are carried out at a cooling rate which lies between 40 Kelvin/second and 150 Kelvin/second. 
     
     
       3. The method according to  claim 1 , wherein during the first annealing process (S 4 . 1 ) and during the second annealing process (S 4 . 2 ) the heating (E 1 ; E 2 ) is carried out at a heating rate which lies between 4 Kelvin/second and 50 Kelvin/second. 
     
     
       4. The method according to  claim 1 , wherein the alloy additionally comprises:
 a silicium fraction (Si), 
 an aluminium fraction (Al), and 
 a chromium fraction (Cr), 
 
       wherein the following relationship between the silicium fraction (Si), aluminium fraction (Al) and chromium fraction (Cr) holds: 0.3 wt. %≤Si+Al+Cr≤3 wt. %. 
     
     
       5. The method according to  claim 4 , wherein
 the chromium fraction (Cr) is always less than 0.4 wt. % and 
 the silicium fraction (Si) lies between 0.25 and 0.7 wt. %. 
 
     
     
       6. The method according to  claim 5 , wherein the silicium fraction (Si) lies in the range of 0.3≤Si≤0.6. 
     
     
       7. The method according to  claim 4 , wherein the following relationship between the silicium fraction (Si), aluminium fraction (Al) and chromium fraction (Cr) holds: 1.2 wt. %≤Si+Al+Cr≤2 wt. %. 
     
     
       8. The method according to  claim 1 , wherein the alloy composition additionally comprises a nitrogen fraction (N) which lies in the range between 0.004 wt. % and 0.012 wt. %. 
     
     
       9. The method according to  claim 8 , wherein the nitrogen fraction (N) lies in the range between 0.004 wt. % and 0.006 wt. %. 
     
     
       10. The method according to  claim 1 , wherein during the first annealing process (S 4 . 1 ) the cooling (A 1 ) of the steel product is carried out so that the course of the temperature (T) of a corresponding cooling curve plotted over the time (t) passes through a region of bainite formation ( 50 ). 
     
     
       11. The method according to  claim 1 , wherein by admixing or adding silicium (Si) and aluminium (Al) a region of bainite formation ( 50 ) during cooling (A 1 ) of the steel product is shifted in a direction of a more rapid cooling. 
     
     
       12. The method according to  claim 1 , wherein the first time period (Δ 1 ) lies in the range of 3≤Δ 1 ≤10 minutes. 
     
     
       13. The method according to  claim 12 , wherein the first time period (Δ 1 ) lies in the range of 4≤Δ 1 ≤5 minutes. 
     
     
       14. The method according to  claim 1 , wherein the second time period (Δ 2 ) is in the range of 3≤Δ 2 ≤5 hours. 
     
     
       15. The method according to  claim 14 , wherein the second time period (Δ 2 ) is in the range of 3.5≤Δ 2 ≤4.5 hours.